Most oil and gas wells eventually require some form of stimulation to enhance hydrocarbon flow and make or keep them economically viable. The servicing of the oil and gas wells to stimulate production requires the pumping of fluids under high pressure. The fluids are generally corrosive and abrasive because they are frequently laden with corrosive acids and abrasive proppants such as sharp sand. Consequently, such fluids can cause irreparable damage to wellhead equipment if they are pumped directly through the spools and the various valves that make-up the wellhead. To prevent such damage, wellhead isolation tools have been used and various configurations are known. Examples of such tools are taught in at least the following patents and patent application:
U.S. Pat. No. 3,830,304 to Cummins;
U.S. Pat. No. 4,241,786 to Bullen;
U.S. Pat. No. 4,632,183 to McLeod;
U.S. Pat. No. 4,111,261 to Oliver;
U.S. Pat. No. 4,867,243 to Gardner et al.;
U.S. Pat. No. 5,332,044 to Dallas;
U.S. Pat. No. 5,372,202 to Dallas;
Canadian Patent No. 1,277,230 to McLeod;
Canadian Patent No. 1,281,280 to McLeod;
Canadian Patent No. 1,292,675 to McLeod;
Canadian Patent Application No. 2,055,656 to McLeod.
All of the wellhead isolation tools described in the patents and patent application listed above operate on the same general principle. Each tool includes a mandrel which is inserted through the various valves and spools of the wellhead to isolate those components from the elevated pressures and the corrosive and abrasive fluids used in the well treatment to stimulate production. A top end of the mandrel is connected to one or more high pressure valves through which the stimulation fluids are pumped. A bottom end of the mandrel includes a packoff assembly for achieving a fluid seal with the production tubing in the well. The mandrel is inserted down through the wellhead to the extent that it enters a top of the production tubing string where the packoff assembly seals against the inside of the production tubing, so that the wellhead is completely isolated from the stimulation fluids.
The mandrel for a wellhead isolation tool must be constructed to withstand high pressures at least about 10,000 psi. The packoff assembly that is bonded to the bottom end of the mandrel and seals against the inside of the production tubing limits the internal diameter of the mandrel and, consequently, the flow rate at which stimulation fluids are pumped through the mandrel is limited. For example, the maximum internal diameter for a mandrel of any one of the wellhead isolation tools described in the patents and patent application listed above is about 1.5" (3.8 cm) when designed for use with a wellhead and a production tubing of standard dimensions. If the stimulation fluids are pumped through a mandrel of that size at 200 feet per second, the fluid flow rate is about 26 barrels per minute (BPM).
Wellhead isolation tools having a packoff assembly that seals against the inside of the production tubing also suffer from other drawbacks. The packoff assembly has a tendency to catch on constrictions as it is inserted through the wellhead, because the packoff assembly that leads the way through the wellhead, is larger than the mandrel, and has a leading edge of rubberized sealing material that seals against the inside of production tubing. In addition, the joint between the mandrel and the packoff assembly creates eddies in the production stimulation fluids which cause washout in the area of the joint.
To overcome the drawbacks of the wellhead isolation tools described in the above-listed prior art, Applicant describes an improved mandrel for a wellhead isolation tool in his co-pending U.S. patent application Ser. No. 08/837,574 filed on Apr. 21, 1997 and entitled APPARATUS FOR INCREASING THE TRANSFER RATE OF PRODUCTION STIMULATION FLUIDS THROUGH THE WELLHEAD OF A HYDROCARBON WELL, the entire specification of which is incorporated herein by reference. The apparatus described in this patent application includes a mandrel for a wellhead isolation tool, and a tubing hanger for use in conjunction with the mandrel. The mandrel includes an annular seal bonded to the outside wall above the bottom end of the mandrel. The annular seal cooperates with the sealing surface in the top end of the tubing hanger to isolate the wellhead equipment from the high pressures and corrosive and abrasive materials pumped into the well during a well treatment to stimulate production. The novel construction for the mandrel and the tubing hanger eliminates the requirement for a packoff assembly attached to the bottom of the mandrel and thereby permits the mandrel to have a larger internal diameter for increasing the transfer rate of production stimulation fluids through the wellhead. However, the annular seal of the mandrel is not adapted to cooperate with a standard tubing hanger. Consequently, a special tubing hanger is required if the mandrel is to be used for wellhead isolation.
It is desirable to further improve wellhead isolation tools to permit a high flow rate during a well treatment to stimulate production, without a requirement for a special tubing hanger so that substantially any well can be treated to stimulate production.